enter image description here this is the basic shematic, i added a freewheel diode with the connector of the motor .

Im driving a DC motor 24DC 1A, im using PWM 500Hz from microcontroller and MOSFET IRF740 with a driver TLP250 and of course the freewheel diode in parallel with the motor.

the problem is that i don't have a clean zero state when the MOSFET is in conduction as you can see in the picture, and because of that he's overheating, but when im using a resistive load i have a very good commutation.

So the problem is because of the inductive load, what should i do to get rid of it ? signal between Drain and source

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    \$\begingroup\$ Could you post a circuit and point out your probe location? \$\endgroup\$ – slightlynybbled Apr 6 '16 at 23:10
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    \$\begingroup\$ Furthermore, you're not showing your scope settings. Also, those noisy OFF state voltages are very peculiar. How about showing your gate voltages? And finally, SHOW YOUR SCHEMATIC! \$\endgroup\$ – WhatRoughBeast Apr 7 '16 at 0:53
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    \$\begingroup\$ @Asmyldof it was resistor 12Ohm, the voltage for the gate is 15v and i tried with 18v it is the same thing. \$\endgroup\$ – lawir Apr 8 '16 at 14:52
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    \$\begingroup\$ @GregoryKornblum the load is a DC motor maximum current 1A, Rdson=0.5ohm P= 0.5W so technically the MOSFET won't heat up. and for the PWM i will change it of course in the real application probably 4 or 5Khz \$\endgroup\$ – lawir Apr 8 '16 at 18:55
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    \$\begingroup\$ @WhatRoughBeast yes of course im using 24VAC with rectifiers and caps. and sorry i don't have yet the right to post more than 2 links that's why i didn't post the gate signal, but i check it and it doesn't problem i got rectangular signal with 15V and 0V. \$\endgroup\$ – lawir Apr 8 '16 at 19:00


Nice, glad it help. So let's summarize.

  1. The PWM frequency should be calculated according to motor time constant (L/R). You didn't measure ohmic resistance, so my educated guess is that it's around 5 ohms.

Wht happens when PWM frequency is low? Motor current reaches very high value despite your duty cycle.

I would argue that 32khz is a little high- high end servo work with such frequency. But it's definitely better than 0.5khz.

Now, i still hate your serial gate resistor. Why? Because it makes rise time of your MOSFET longer. During that time the voltage across transistor is not zero (average of half VBUS) and current is full, so power is quite high.

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